Lubricating oil compositions for use in crankcase engine oils comprise a major amount of base oil and minor amounts of additives that improve the performance and increase the useful life of the lubricant. Crankcase lubricating oil compositions conventionally contain polymeric components that are used to improve the viscometric performance of the engine oil, i.e., to provide multigrade oils such as SAE 5W-30, 10W-30 and 10W-40. These viscosity performance enhancing material, commonly referred to as viscosity index (VI) improvers, can effectively increase the viscosity of a lubricating oil formulation at higher temperatures (typically above 100° C.) without increasing excessively the high shear rate viscosity at lower temperatures (typically −10 to −15° C.). These oil-soluble polymers are generally of higher molecular weight (>100,000 Mn) compared to the base oil and other components. Well known classes of polymers suitable for use as viscosity index improvers for lubricating oil compositions include ethylene α-olefin copolymers, polymethacrylates, diblock copolymers having a vinyl aromatic segment and a hydrogenated polydiene segment, and star copolymers and hydrogenated isoprene linear and star polymers.
Viscosity index improvers for lubricating oil compositions advantageously increase the viscosity of the lubricating oil composition at higher temperatures when used in relatively small amounts (have a high thickening efficiency (TE)), provide reduced lubricating oil resistance to cold engine starting (as measured by “CCS” performance) and be resistant to mechanical degradation and reduction in molecular weight in use (have a high shear stability index (SSI)). It is also preferred that the viscosity index improver to display soot-dispersing characteristics in lubricating oil compositions. Further, as viscosity index improving polymers are often provided to lubricant blenders as a concentrate in which the viscosity index improving polymer is diluted in oil, which concentrate is then blended into a greater volume of oil to provide the desired lubricant product. Therefore, it is further preferred that viscosity index improving polymers can be blended into concentrates in relatively large amounts, without causing the concentrate to have an excessively high concentrate the kinematic viscosity. Some polymers are excellent in some of the above properties, but are deficient in one or more of the others.
It would be advantageous to be able provide lubricating oil compositions that simultaneously provide high overall viscometric performance, and soot dispersancy.
PCT Publication WO 96/17041, Jun. 6, 1996, discloses certain blends of star-branched styrene-isoprene polymers and ethylene α-olefin copolymers. The publication describes the addition of a an amount of the ethylene α-olefin copolymer to the star-branched styrene-isoprene polymer as being effective to improve the dimensional stability of the star branched polymer so that the star branched polymer can be formed as a stable, solid bale.
U.S. Pat. No. 4,194,057, Mar. 18, 1980, discloses viscosity index improving compositions containing a combination of a certain class of relatively low molecular weight vinyl aromatic/conjugated diene diblock copolymers and ethylene α-olefin copolymer. The patent describes the specified class of vinyl aromatic/conjugated diene diblock copolymer as being relatively insoluble in oil and that blending with ethylene α-olefin copolymer improves solubility and allows for the formation of polymer concentrates.
PCT Publication WO 2004/087849, Oct. 14, 2004, discloses a viscosity index improver composition containing a blend of a select class of high ethylene content ethylene α-olefin copolymer, and vinyl aromatic/diene diblock copolymer, in certain proportions, which are describes as providing good low temperature performance and durability.